Mail Transport Agents; Sendmail - Red Hat ENTERPRISE LINUX 5 - DEPLOYMENT Deployment Manual

24.3. Mail Transport Agents

Red Hat Enterprise Linux includes two primary MTAs, Sendmail and Postfix. Sendmail is configured
as the default MTA, although it is easy to switch the default MTA to Postfix.

24.3.1. Sendmail

Sendmail's core purpose, like other MTAs, is to safely transfer email among hosts, usually using
the SMTP protocol. However, Sendmail is highly configurable, allowing control over almost every
aspect of how email is handled, including the protocol used. Many system administrators elect to use
Sendmail as their MTA due to its power and scalability.
24.3.1.1. Purpose and Limitations
It is important to be aware of what Sendmail is and what it can do, as opposed to what it is not. In
these days of monolithic applications that fulfill multiple roles, Sendmail may seem like the only
application needed to run an email server within an organization. Technically, this is true, as Sendmail
can spool mail to each users' directory and deliver outbound mail for users. However, most users
actually require much more than simple email delivery. Users usually want to interact with their email
using an MUA, that uses POP or IMAP, to download their messages to their local machine. Or, they
may prefer a Web interface to gain access to their mailbox. These other applications can work in
conjunction with Sendmail, but they actually exist for different reasons and can operate separately
from one another.
It is beyond the scope of this section to go into all that Sendmail should or could be configured to
do. With literally hundreds of different options and rule sets, entire volumes have been dedicated
to helping explain everything that can be done and how to fix things that go wrong. Refer to the
Section 24.7, "Additional Resources"
This section reviews the files installed with Sendmail by default and reviews basic configuration
changes, including how to stop unwanted email (spam) and how to extend Sendmail with the
Lightweight Directory Access Protocol (LDAP).
24.3.1.2. The Default Sendmail Installation
The Sendmail executable is /usr/sbin/sendmail.
Sendmail's lengthy and detailed configuration file is /etc/mail/sendmail.cf. Avoid editing
the sendmail.cf file directly. To make configuration changes to Sendmail, edit the /etc/
mail/sendmail.mc file, back up the original /etc/mail/sendmail.cf, and use the following
alternatives to generate a new configuration file:
• Use the included makefile in /etc/mail (make all -C /etc/mail) to create a new /etc/
mail/sendmail.cf configuration file. All other generated files in /etc/mail (db files) will be
regenerated if needed. The old makemap commands are still usable. The make command will
automatically be used by service sendmail start | restart | reload if the make
package is installed.
• Alternatively you may use the included m4 macro processor to create a new /etc/mail/
sendmail.cf.
More information on configuring Sendmail can be found in
Configuration Changes".
Various Sendmail configuration files are installed in the /etc/mail/ directory including:
for a list of Sendmail resources.
Section 24.3.1.3, "Common Sendmail
Mail Transport Agents
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